Conventional industrial processes for preparing hydrogen peroxide generally include, for example, an electrolytic method using acidic ammonium sulfate, an autooxidation method using anthraquinones, and an oxidation method using isopropyl alcohol. On the other hand, various methods have been proposed for synthesizing hydrogen peroxide directly from hydrogen and oxygen.
A method has been known for preparing hydrogen peroxide by a catalytic reaction of hydrogen and oxygen generally at atmospheric pressure in an aqueous medium containing a platinum-group catalyst. According to this method, however, the resulting hydrogen peroxide further reacts with hydrogen to decompose to water, and the concentration of the hydrogen peroxide in the aqueous medium of the reaction system is as low as about 0.1% by weight (about 0.05 mole%). Accordingly, this method can hardly be expected to be commercially feasible.
In an attempt to overcome the above problem, a method has previously been proposed in which the reaction is carried out in the presence of a stabilizer for inhibiting the decomposition of hydrogen peroxide prepared from hydrogen and oxygen. As such a stabilizer, U.S. Pat. No. 3,361,533 discloses the use of an acid and a non-acidic oxygen-containing organic compound such as acetone or an alcohol, U.S. Pat. No. 3,336,112 discloses the use of a salt of condensed phosphoric acid, and Canadian Pat. No. 791,614 discloses the use of boric acid. The methods disclosed in these patents, however, afford hydrogen peroxide in a concentration of as low as less than about 3% by weight, and are therefore unsatisfactory.
The present inventors previously suggested in U.S. patent application Ser. No. 589,776 filed June 23, 1975, now U.S. Pat. No. 4,009,252 issued Feb. 22, 1977, a process for preparing hydrogen peroxide directly from hydrogen and oxygen which can afford hydrogen peroxide in high concentrations by using a combination of specific process conditions in the absence of a stabilizer. According to this process, the reaction is carried out in the presence of not more than 30 mg, per 100 ml of the aqueous medium, of a platinum-group catalyst calculated as the metal while maintaining the partial pressure of hydrogen and the partial pressure of oxygen in the gaseous phase of the reaction system at at least 0.5 atmosphere and at least 1.0 atmosphere respectively with the ratio of the partial pressure of oxygen to that of hydrogen adjusted to 1.5:1 to 20:1. This process successfully affords hydrogen peroxide in a concentration of as high as 12.8%. This prior application discloses that good results are obtained when the platinum-group catalyst is added to the aqueous medium after hydrogen has been introduced into the aqueous medium to adjust the partial pressure of hydrogen in the gaseous phase of the reaction system to at least 0.03 atmosphere.
U.S. Pat. No. 4,009,252 has furnished a superior process for obtaining hydrogen peroxide of high concentrations. It has been found however that when hydrogen peroxide is present in an amount above a certain limit in the aqueous medium or reaction mixture (for example, in the continuous process using the reaction mixture), the activity of the catalyst is reduced, and as a result, the concentration of the hydrogen peroxide decreases. This phenomenon begins to be perceptible when the amount of hydrogen peroxide present in the reaction mixture becomes at least 0.5% by weight, and develops to a considerable degree when the amount becomes 2% by weight or more. If the amount exceeds 5% by weight, the phenomenon becomes serious. Further investigation of this phenomenon led to the discovery that hydrogen peroxide present in advance to the reaction mixture exerts an undesirable action of reducing the activity of the catalyst to be added, but that hydrogen peroxide formed as a result of reaction and being present in the reaction mixture does not reduce the activity of the catalyst. For example, when an autoclave-type reactor is charged with an aqueous medium free from hydrogen peroxide, and then hydrogen and oxygen are introduced into the aqueous medium and reacted batchwise in the presence of a platinum-group catalyst, hydrogen peroxide is not present in the reaction mixture at the start of the reaction, but with the progress of the reaction, it is accumulated gradually in the reaction system. The activity of the catalyst is scarcely reduced by the hydrogen peroxide accumulated by reaction after the addition of the catalyst even when the concentration of hydrogen peroxide exceeds 5% by weight. In contrast, in a continuous process in which the reaction mixture is recycled, and a fresh supply of catalyst is added to the reaction system while recovering the resulting hydrogen peroxide and the spent catalyst, the hydrogen peroxide present in the reaction system before the addition of the catalyst acts on the catalyst freshly supplied to reduce its activity. The cause of this phenomenon has not yet been elucidated, but it is at least certain that this phenomenon becomes a serious drawback toward the preparation of hydrogen peroxide in high concentrations by the continuous process.
The present invention intends to solve the above-mentioned problem, and has for its object the provision of a novel process for preparing hydrogen peroxide directly from hydrogen and oxygen, which can afford hydrogen peroxide in high concentrations by preventing the reduction of the catalytic activity even when a detrimental amount of hydrogen peroxide is present in the aqueous medium. The process of the invention is especially suitable for continuously producing hydrogen peroxide in a concentration of as high as 5 to 13% by weight from hydrogen and oxygen.